Vanadium dioxide microparticles, method for preparing same, and use thereof, in particular for surface coating

ABSTRACT

Vanadium dioxide particles having formula V1-xMxO2, wherein 0&lt;=x&gt;=0.05 and M is a doping metal, and being characterized in that they have a particle size of less than 10 mum, a method for preparing same, and the use of said microparticles, in particular for surface coating, are disclosed.

The object of the present invention is vanadium dioxide microparticles,a method for preparing said microparticles and their applications,notably for surface coatings in which they are incorporated.

In a first aspect, the invention relates to microparticles of vanadiumdioxide of formula V_(1−x)M_(x)O₂ in which 0≦x≦0.05 and M is a dopingmetal, said microparticles having a particle size of less than 10 μm,notably less than 5 μm, preferably in the order of 0.1 to 0.5 μm.

The doping metal may be selected from transition elements which offer anionic beam greater than that of vanadium such as for example Nb or Ta oran electronic contribution such as for example Mo or W, W and Mo beingpreferred.

In a preferred aspect, the microparticles according to the invention areconstituted of doped vanadium dioxide of formula V_(1−x)W_(x)O₂ in whichx is between 0 and 0.02.

The vanadium dioxide microparticles according to the invention maynotably be used in the technical sector of coating compositions intendedto be essentially deposited in thin layers in the form of a film or aleaf, such as paints, varnishes and any other type of coating that maybe deposited in successive layers.

The aim of the invention is therefore to use the vanadium dioxidemicroparticles described above for carrying out an <<intelligent>>material which automatically reduces the transmission of solar rays inthe domain of infra-red rays, when the material reaches a giventemperature level. It is thus possible to benefit from the energy of theinfra-red rays below the fixed temperature and to eliminate theexcessive heating above this temperature.

One of the principal applications of the vanadium dioxide microparticlesaccording to the invention is their use in coatings intended to beaffixed on the facades of buildings exposed to bad weather. The darkcoloured coatings exposed to the sun's rays heat up much more than thoseof light colour. They therefore undergo expansion-contraction cycles ofvery high amplitude which cause a premature degradation of the coatingsheet. It is therefore not possible at the present time to guarantee adark paint whose luminous luminance is lower than 35%.

This phenomenon may be limited by the addition of a vanadium dioxidepigment to the paint whose fixed transition temperature should be in theorder of 25° C. for example.

Another application is that of the protection of transparent ortranslucent surfaces which must allow visible rays to pass through them,such as in greenhouses, verandas, housing glazings, but whose internaltemperature needs to be controllable, such a use may also be envisagedwithin the context of glazings and coachwork of cars and all othertransport vehicles.

In summer, by reducing the entry of incident solar energy intobuildings, the coating enables reducing the needs for air-conditioningand, on the other hand, in winter, the coating limits the dissipation ofheat towards the exterior. Thus the coating advantageously allows aneconomy in energy.

One of the objects of the present invention is in fact thecontrollability of the transfer and the absorption of calorific energyat the surface of a wall without necessitating specifically transformingor treating the material thereof, but by depositing a coating followingany known method, such as is practised with paints, it being possiblefor said coating according to the invention to be itself such a paint,enabling an economic implementation and manufacture.

Now, various ionic or molecular compounds are known which, under theeffect of a variation of temperature, can change the optical properties,principally the colour, linked to a change of electronic structure: suchcompounds are called <<thermochromic>> compounds. By extension, acompound may also be called <<thermochromic>> which has the property ofabsorbing and/or reflecting different types of rays according totemperature due to a change in electronic structure. Vanadium dioxidehas thus been studied for several years which has a structuraltransition at a temperature T_(t)=341 K or 68° C.: below T_(t) thecrystalline structure is monoclinic, whereas above T, the structure isrutile. This transition is associated with a sudden change in theelectronic properties: the compound thus passes into the insulatingstate when the temperature is lower than T_(t) and into the metallicstate when the temperature is greater than T_(t); optically, this changemanifests itself as deep modifications of the near and far infra-redabsorbance and reflection properties.

In the rest of the description, the designation <<vanadium dioxide>>shall comprise vanadium dioxide commonly named VO₂ or V₂O₄.

Various studies have recently been carried out on this compound, such asthose that may be picked out in the publications S. M. Babulanam, Mat.Opt. Sol. Light Techn. 692 (1986) 8 and J. C. Valmalette, Sol. EnergyMater 33 (1994) 135. Studies have therefore been conducted on thinlayers of vanadium dioxide deposited on various substrates: they havenotably revealed the practical interest of the development of a materialwhich is transparent to light but which only allows the infra-red partof the solar spectrum to pass through at low temperature. From this, thevanadium dioxide seems at the present time to be the only compound forwhich the transition is situated in a range of temperature andwavelengths suitable to the thermal regulation of the housing.

Moreover, this compound has the additional advantage of being able toundergo chemical substitutions with appropriate atoms such as definedfurther on and enabling a displacement of the temperature T_(t)towardslower temperatures.

Thus, many tests and researches have been developed to create thinlayers of vanadium dioxide deposited on substrates, notably with theview to studying the optical transmittance in the visible and the nearinfra red; for this, various depositing techniques have been envisaged,such as cathodic spraying under vacuum, evaporation under beam, vapourphase chemical deposits and the <<sol-gel>> process.

According to the <<sol-gel>> process, vanadium dioxide is prepared fromtetravalent vanadium by dissolution in a solvent, hydrolysis andcondensation in order to gradually form a sol, then, by evaporating thesolvent, forming a gel which is then submitted to a thermal treatment togive VO₂, under a finely controlled atmosphere.

It is possible to directly form a VO₂ film on a substrate, by soaking anappropriate substrate in the sol. The gel is thus formed directly on thesubstrate. Such a process of moistening or <<dip-coating>> is notablydescribed in the U.S. Pat. No. 4,957,725.

It is however difficult to control the quality of the final filmdeposited, i. e. to place the complete piece or even its surface at hightemperature in a uniform way and to control the interactions between thesupport and the gel thus deposited, etc . . . Thus, on the one hand,such methods which do not apply to the materials already constituted donot really enable an application on very large surfaces such as can bedone with a surface coating composition such as paint and, on the otherhand, the results obtained are neither repetitive nor reliable.Moreover, it is a very costly process when it comes to large surfaces.

Processes via the dry route generally exist which are very long (in theorder of fifteen days) and are therefore very costly, which only enableobtaining molecules-grains in the order of 30 microns and more, which isnot compatible with an incorporation into a paint without modifying thecolour of it, which does not allow a homogeneous mixture and which doesnot bring about the property of optical transmission.

The problem posed is therefore one of being able to obtain a powder oflow particle size which essentially comprises vanadium dioxide which isdoped or not notably with tungsten which may notably be able to beincorporated in a liquid or viscous support with the view to obtaining asurface coating.

In a second aspect, the invention therefore relates to a method ofobtaining microparticles of vanadium dioxide of formula V_(1−x)M_(x)O₂in which M is a doping metal and 0≦x≦0.02, by pyrolysis of doped ornon-doped ammonium hexavanadate, characterised in that said pyrolysis iscarried out at a temperature between about 400° C. and about 650° C.,with a temperature increase rate of at least 100° C./min, and in thatthe gases resulting from said pyrolysis are kept in confinement and indirect contact with the reaction medium for a period of time of at least½ hour, preferably 1 hour.

The use of ammonium hexavanadate (NH₄)₂V₆O₁₆ is known in industry forthe manufacture of V₂O₅ commonly used as catalyst, but in which thetetravalent vanadium is considered as a non-catalytic impurity whoseremoval is sought. The tests which have been able to be done with thisprecursor in order to also obtain vanadium dioxide alone have not led toanything since V₂O₃ was obtained and every publication up to the presentday maintains that it was not possible to obtain pure vanadium dioxide.

The objects, advantages and features of this invention will be moreclearly appreciated from the following detailed description, when takenin conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing an oven for carrying out oneembodiment of the present invention;

FIG. 2 is a SEM photomicrograph of grains prepared according to oneembodiment of the present invention;

FIG. 3 is an x-ray spectrograph of the product prepared in Example 1;

FIGS. 4a-b are infrared spectrographs of the product prepared in Example1 recorded at a temperature that is above 68° C. (FIG. 4a) and below 68°C. (FIG. 4b);

FIG. 5 is a SEM photomicrograph of the product prepared in Example 1;

FIGS. 6a-b are Infrared spectrographs of the product prepared in Example4 recorded at a temperature that Is above 68° C. (FIG. 6ae) and below68° C. (FIG. 6b);

FIG. 7 is a SEM photomicrograph of the product prepared in Example 4;

FIGS. 8a-b are absorption spectra of the film prepared In Example 7during heating (FIG. 8a) and during cooling (FIG. 8b); and

FIG. 9 is a schematic diagram showing a device for measuring solar flux.

The implementation of the characteristic conditions of the method ofpyrolysis according to the invention, namely:

a temperature increase rate of at least 100° C./min, preferably 200°C./min or 300° C./min, and

the non-evacuation of the gases resulting from the thermal decompositionof ammonium hexavanadate, notably NH₃, which are kept in confinement andin contact with the reaction medium at least 5 minutes, preferablybetween ½ hour and 2 hours, and at the most throughout the wholeduration of the synthesis, allows obtaining a complete reaction withoutany formation of residual V₂O₅ according to the following reactionscheme:

(NH₄)₂V₆O₁₆→NH₃+V₂O₅→VO₂+H₂O+N₂

It has in fact been possible to notice that at a fast pyrolysis rate a<<flash>> reaction is created which produces N₂O which slowly decomposesby reacting with excess NH₃ forming H₂O and N₂.

Furthermore, to this day, in most existing ovens, either the reactionplace is brushed out, carrying away the NH₃ gas produced and thusstopping the reduction which produces only V₆O₁₃ and does not allowgoing as far as vanadium oxide, VO₂, V₂O₄; or on the contrary othermethods which add NH₃ by circulation, which creates too significant areduction and leads the reaction to obtain V₂O₃ and a mixture of severalvanadium oxides.

Advantageously, the gases resulting from the thermal decomposition ofammonium hexavanadate are collected in a gas bag under slight pressure,for example about 0.5 bar, placed preferably at a level higher than thatof the reactor.

The pyrolysis temperature must be between about 400° C. and about 650°C., preferably 635° C. If the temperature is above about 650° C., theV₂O₅ present in the reaction medium risks melting before it reacts; Onthe other hand, a reaction temperature below about 400° C. leads tonon-thermochromic VO₂ (B).

In the case of the preparation of doped vanadium dioxide particles, thisduration must also be fixed in such a way as to obtain a dopinghomogeneity whilst preventing the growth of the grains by optimisationof the temperature-time compromise.

For example, for a doping rate of 5% W:

temperature 600° C. 650° C. 700° C. minimum time 116 hours  3 hours 1hour maximum time 60 hours 12 hours 6 hours

If it is desired to prepare vanadium dioxide microparticles having astructural transition temperature different from 68° C. (correspondingto pure vanadium oxide), it is necessary to dope it with a substitutionproduct whose stable valency must be greater than 4.

In a preferred aspect, a metal selected from Nb, Ta, Mo and W will beused as substitution product, W and Mo being preferred.

Substitution with tungsten (W) allows obtaining a final product whichhas a significant temperature variation gradient as a function ofpercentage substitution: a significant gradient allows in fact coveringquite a large range of temperatures. Thus, for examples of the value ofx above, a transition temperature is obtained of:

x = 0% 1% 2% 3% T_(t) = 68° C. 40° C. 12° C. −16° C.

In a preferred aspect of the method, ammonium hexavanadate is thereforeused which is doped with a metal selected from Nb, Ta, Mo and W, W andMo being preferred.

In the following part of the description, <<doping by a metal>>, themetal being such as defined above, means doping carried out by using themetal in the pure form or in the form of a compound containing same,such as notably a tungstate or a molybdate.

The ammonium hexavanadate used in the method according to the inventionis commercially available. It may also be prepared in a known mannerfrom ammonium metavanadate.

When it is desired to prepare doped vanadium dioxide microparticles,either ammonium hexavanadate can be doped, or the doping metal can beincorporated during the synthesis of the hexavanadate from ammoniummetavanadate.

The use of tungsten as substitution product is also advantageous insofaras the ammonium tungstate is very soluble in water.

Notably, when it is desired to incorporate tungsten into the ammoniumhexavanadate already synthesised, the ammonium tungstate may easily beplaced in solution in water with the ammonium hexavanadate, with aminimum of moistening of 20% by weight in order to obtain a homogeneousground paste.

The chemical substitution or doping is thus carried out by pyrolysis ofthe mixture of the ammonium tungstate and hexavanadate precursorsaccording to the following reduction and substitution reaction:

(1−x) (NH₄)₂V₆O₁₆ +x/2(NH₄)₂H₂W₁₂O₄₀ , yH₂O→6(V_(1−x)W_(x))O₂ +nN₂ +mH₂O

The choice of x for a homogeneous result is an exact stoichiometriccalculation which enables obtaining the desired temperature variationwith respect to the transition temperature of 68° C. of vanadiumdioxide: it may be noted for this that the gradient δ_(t)/dx=−28 in 10²K/mole, be it in fact for x=0.01 or 1%, δ_(t)=−28° C.

According to an advantageous aspect of the method according to theinvention, the ammonium hexavanadate is submitted before pyrolysis to adegassing at a temperature lower than the decomposition temperature ofammonium hexavanadate, notably lower than about 230° C., and preferablyin the order of 200° C., and by carrying out a first pumping undervacuum for at least one minute, for example 15 minutes.

At the end of the pyrolysis, the vanadium dioxide obtained mayadvantageously be submitted to an annealing step under inert gas, at atemperature of at least 600° C., for a period of time of at least 1hour, for example 5 hours.

For example, this annealing step may be carried out at 600° C. for 14hours. At 800° C. for 5 hours, the grain growth is greater than 5 μm.

In a preferred aspect of the method according to the invention, andoptionally after the above-mentioned annealing step, the vanadiumdioxide will be cooled under inert gas to a temperature of about 120° C.The cooling rate may be for example about 150° C./min to 250° C./min.

Nitrogen or argon for example may be used as inert gas.

During the exit of the vanadium dioxide into the free air, thetemperature must be at the most about 100° C. in order to prevent ittaking up water and therefore risking surface re-oxidation.

An example of an oven which allows carrying out the method according tothe invention is represented in FIG. 1 in which, as well as the assemblyof a mobile tubular oven (2) on a rail (3) with respect to the treatmentchamber (1) constituted of a quartz tube which enables obtaining therapid rate of obtaining the temperature desired, is represented by a gasbag (4) which allows keeping the gases resulting from the thermaldecomposition of ammonium hexavanadate, notably NH₃, in confinementabove the grains to be treated in the chamber (1). This allowsmaintaining a partial ammonia pressure sufficient so that the reductionreaction to VO₂ be total. The heavier gas N₂O is carried away towardsthe bottom in said gas bag (4), which allows increasing and optimisingthe purity of the compounds obtained.

The valve (5) which allows the eventual exit of the gases, the argon andnitrogen feeding cylinders (6) and (7), a vacuum pump (8) and thepressure indication dial (9) are also shown in FIG. 1.

Thus, according to the method of the invention, vanadium dioxidemicroparticles may be obtained which have a particle size lower than 10μm, notably lower than 5 μm, preferably in the order of 0.1 to 0.5 μm.

Certain grains may be from 2 to 10 μm after reaction but may be easilybroken up by grinding; they are all in fact advantageously in the formof <<pre-cut >> platelets when they are prepared by the method of thepresent invention, as is shown by the sweeping electron microscopephotograph (SEM) represented in FIG. 2, whilst in prior art methods,such as recalled above, even when pure vanadium dioxide is obtained, itis essentially a matter of massive monocrystals in the order of 30 μmwhich are very difficult to break up.

Thus, in a preferred aspect of the method, the vanadium dioxide obtainedafter pyrolysis, said pyrolysis optionally being followed by anannealing process and/or cooling process such as mentioned above, issubmitted to a moist grinding. Said grinding may be carried out forexample in a spinning zircon ball grinder at more than 3000 turns/minfor a period of time of less than or equal to 2 hours.

Thus, after treatment by grinding, immediately or afterwards, in thecontext of its incorporation in a surface coating composition ofvanadium dioxide obtained according to the invention, the whole of itmay be brought up to a range of particle size of 0.1 to 0.5 μm: suchbase particles enable obtaining then a transparent film which respondsto the objectives sought-after and satisfy the applications of theinvention, either by mixing with a paint coloured by any pigment,without significantly changing the colour, or by incorporating into avarnish in keeping the transparency thereof.

In a subsequent aspect, the invention therefore relates to the use ofthe vanadium dioxide microparticles according to the invention for thepreparation of surface coating compositions.

The incorporation of vanadium dioxide in the surface coatingcomposition, notably a paint or a varnish, may be by any known methodsuch as impasting, introduction with stirring, of the doped or non-dopedvanadium dioxide and a dispersing agent to help its dispersion and tostabilise it in this form. A grinding is optionally carried out toreduce the size of the particles, such as for example in a spinningzircon ball grinder spinning at more than 3,000 turns per minute for 2hours, which allows breaking the vanadium dioxide microparticles upwhich would eventually be bigger than 0.1-0.5 μm, said grinding beingcarried out, as mentioned above, on the vanadium dioxide beforeincorporation into the surface coating composition.

In a subsequent aspect, the invention also relates to surface coatingcompositions which contain vanadium dioxide microparticles described inthe present application.

The invention is illustrated by the Examples below which are in no waylimiting.

EXAMPLE 1 Synthesis of Non-doped Vanadium Dioxide

I. Production of the Ammonium Hexavanadate Precursor (AHV)

20 g of ammonium metavanadate (AMV) (Aldrich Ref. 20,555 9; purity: 99%;M: 116.78) are introduced into a 250 ml beaker. The beaker is placed ona heating plate. A few drops of water are added with stirring so as toform a fluid paste in order to initiate the dissolution of the AMV. Thebeaker is heated at 55° C. ±5° C. in maintaining stirring. A 1N solutionof hydrochloric acid is then added dropwise (at the start) whiststirring and keeping the temperature constant.

The pH is checked in order to regulate the rate of acid addition and toprevent sudden drops in pH. The total duration of this step is greaterthan a half hour.

For a volume of acid added of about 110 cm³ (about 2 moles of acid for 3moles of AHV), the pH drops suddenly without coming back up. Beyondthis, the pH reaches a limiting value, the product obtained is a yelloworange paste, the addition of 1N hydrochloric acid may cease after 120cm³.

The yellow orange precipitate obtained is then rinsed with water(slightly acidic to prevent re-dissolution) in a filter crucible of No.5 porosity by drawing under vacuum, then dried in air at 200° C. in anoven for 24 hours. 17 g of AHV are obtained.

Weight yield (expressed with respect to the initial mass of AMV)=molaryield (expressed in moles of vanadium): greater than 99%

II Pyrolysis of the Precursor

a. Prior degassing of the AHV:

2 g of AHV are deposited in an aluminium basket in zone A (T=200° C.) ofthe oven. A first pumping under vacuum is carried out for 15 minutes.

b. Thermal decomposition of the AHV and formation of VO₂.

The basket is then placed directly in the second zone (B) of the ovenwherein the temperature is 600±5° C. (temperature increase rate of about250° C./min.).

The whole of the gases emitted is recovered in a gas bag under slightpressure of 0.5 bar in direct contact with the reactor and placed at aheight lower than that of the reactor for 1 hour.

c. Cooling and exit from the oven:

The cooling in zone C of the oven is carried out in an atmosphereresulting from the decomposition to a temperature of 120° C. at a rateof about 200° C./min.

Mass of bluish-black powder (VO₂) formed: 1.6680±0.0005 g.

The X-ray spectrum, the IR spectrum and the sweeping electron micrograph(SEM) of the product of Example 1 are represented in FIGS. 3, 4 and 5,respectively. In FIG. 4a, the IR spectrum is recorded at a temperatureabove 68° C. and in FIG. 4b, the IR spectrum is recorded at atemperature below 68° C.

EXAMPLE 2 Synthesis of Non-doped Vanadium Dioxide from an Industrial AHVPrecursor

a. Prior degassing of the AHV:

2 g of AHV (Treibacher (Austria), Ref. AHV Trocken 99%) in an aluminiumbasket in zone A of the oven (T=200° C.). A first pumping under vacuumis carried out for 15 minutes.

b. Thermal decomposition of the AHV and formation of VO₂:

The basket is then placed directly in the second zone (B) of the ovenwherein the temperature is 600±5° C. (temperature increase rate about250° C./min.).

The whole of the gases emitted is recovered in a gas bag under slightpressure and in direct contact with the reactor and placed at a heightlower than that of the reactor for 1 hour.

c. Annealing of the vanadium dioxide:

The sample was left for 14 hours at 600±5° C. in the reactor in thepresence of the decomposition gases.

d. Cooling and exit from the oven:

The cooling was carried out in the atmosphere resulting from thedecomposition to a temperature of 120° C. at a rate of about 200°C./min.

Mass of bluish-black powder (VO₂) formed: 1.6689±0.0005 g.

EXAMPLE 3 Synthesis of Non-doped Vanadium Dioxide from an Industrial AHVPrecursor

An industrial AHV precursor (Treibacher (Austria) Ref. AHV Troken 99%)is used.

a and b. The degassing and the thermal decomposition of the AHV arecarried out as indicated above in Example 2.

c. Annealing of the vanadium dioxide:

The sample was taken out and then put back in the reactor in zone A(T=200° C.). The whole was then first placed under vacuum for 15minutes. The sample was then displaced in the zone B of the oven at thetemperature of 800° C. for 5 hours.

d. Cooling and exit from the oven:

The cooling in zone C of the oven was carried out in the atmosphereresulting from the decomposition to a temperature of 120° C. at a rateof about 200° C./min.

Mass of bluish-black powder (VO₂) formed: 1.6682±0.0005 g.

EXAMPLE 4 Synthesis of Doped Vanadium Dioxide of Formula V_(1−x)M_(x)O₂,with x=0.01 and M=W

I. Production of the AHV Precursor with Doping Agent

The AHV precursor is prepared as indicated above in Example 1, but byadding 0.459 g of ammonium tungstate (Aldrich Ref. 32,238.5; purity:99%; M=265.88) before the addition of 1N hydrochloric acid.

The mass of product obtained is 19.424 g (including ammonium chloride).The product is characterised by an X-ray diffraction diagram and FTIR:very weak bands due to the tungstate in addition to that of AHV.

II. Pyrolysis of the Precursor

a. Prior degassing of the doped AHV.

2.000 g of doped AHV prepared above are deposited in an aluminium basketin zone A (T=200° C.) of the oven. A first pumping under vacuum iscarried out for 15 minutes.

b. Thermal decomposition of the doped AHV and formation of VO₂ dopedwith tungsten.

The basket is then placed directly in the second zone (B) of the ovenwherein the temperature is 600±5° C. (temperature increase rate of about250° C./min.).

The whole of the gases emitted is recovered in a gas bag under slightpressure and in direct contact with the reactor and placed at a heightlower than that of the reactor.

c. Annealing of the vanadium dioxide.

The sample was left at 600° C. for 14 hours.

d. Cooling and exit from the oven.

The cooling in zone C of the oven was carried out in the atmosphereresulting from the decomposition to a temperature of 120° C. at a rateof about 200° C./min.

Mass of bluish-black powder (VO₂) formed: 1.669±0.001 g.

The IR spectrum and SEM photograph of the product of Example 4 arerepresented in FIGS. 6 and 7 respectively. In FIG. 6a, the IR spectrumis recorded at a temperature above 68° C. and in FIG. 6b, the IRspectrum is recorded at a temperature below 68° C.

EXAMPLE 5 Synthesis of Doped Vanadium Dioxide of Formula V_(1−x)M_(x)O₂,with x=0.01 and M=W, from an Industrial AHV Precursor

I. Incorporation of the Doping Agent

20 g of AHV are introduced in a grinder in 25 ml of water in order toform a viscous paste. The paste is then submitted to a first grindingwhose aim is to homogenise the dispersion of the AHV in the aqueousmedium.

Ammonium tungstate is a white powder soluble in water. 0.539 g of it areadded to the grinding paste and the dispersion is continued for severalminutes.

The mixture thus obtained is dried under vacuum or in an oven at 200° C.

II. Pyrolysis of the Doped AHV Precursor

The pyrolysis is carried out under the same implementation conditions asin Example 2 above, in carrying out the annealing step for 5 hours at800° C.

Mass of bluish-black powder (VO₂) formed: 1.670±0.001 g.

EXAMPLE 6 Synthesis of Doped Vanadium Dioxide of Formula V_(1−x)M_(x)O₂,with x=0.02 and M=W

I. Incorporation of the Doping Agent

Carried out as indicated above in Example 5 from 20 g of AHV(Treibacher, Austria, Ref. AHV Troken 99%). The mass of ammoniumtungstate incorporated is 1.089 g.

II. Pyrolysis of the Doped AHV Precursor

The pyrolysis is carried out under the same implementation conditions asin Example 5 above.

Mass of bluish-black powder (VO₂) formed: 1.671±0.001 g.

EXAMPLE 7 Characterisation of the Films and Optical Measurements.

Dry films have been prepared which contain VO₂ doped or not with 1%tungsten (V_(1−x)M_(x)O₂ with x=0.01 and M=W) (solvent phase) in thefollowing manner:

1) Preparation of the Varnish

Empirical formula of the varnish

Plexigum P675 (HULS, 34 (Acrylic copolymer) Germany) Solvanter S340(Elf, 28 Hydrocarbon solvent 100% aromatics) France) White spirit 17%(Elf, 38 Hydrocarbon solvent 17% aromatics) France) 100.00

Solvantar S340 and White Spirit 17% are weighed out into a beaker, thenthe Plexigum P675 is added with stirring and then left to stir untilperfect homogenisation.

2) In Corporation of the VO₂

100 g of varnish are weighed out into a beaker. 1 g of VO₂ (doped ornot) are the added with stirring. Stirring is continued at 1500turns/minute for at least 15 minutes until perfect homogenisation. Thegrinding is carried out with the aid of a glass microball grinder.

3) Application

The coating thus obtained is applied onto a glass plate with the aid ofa manual applicator which enables the deposit of a thickness of 50 μmhumid.

Drying is carried out at ambient temperature.

These films were characterised by the following techniques:

FTIR spectroscopy

optical measurements by light-measuring (measurement of solar flux).

4) Results

a. Characterisation of the transition by FTIR spectroscopy.

The free films are prepared by application on glass, drying andunsticking of the substrate.

The free films were characterised by FTIR in transmission and in ATR(Attenuated Total Reflection). The insulating-metal transition of thedioxide was clearly demonstrated during the heating and cooling by thedisappearance and the reappearance of the absorption bands due to VO₂ atT_(t)=66±2° C. for the film which contains non-doped VO₂ and atT_(t)=±2° C. for a film which contains VO₂ doped with 1% tungsten.

FIGS. 8a and 8 b affixed represent the three-dimensional evolution ofthe absorption bands during the heating of the film (FIG. 8a) thencooling (FIG. 8b) for a film which contains non-doped VO₂. It may beobserved that only the bands of the polymer remain unchanged.

b. Sweeping electron microscopy.

The SEM has enabled

on the one hand to characterise the distribution of the grains of VO₂ inthe dry film,

on the other hand, to carry out a precise measurement of the thickness.

c. Optical measurement by light-measuring.

A device represented in FIG. 9 for measuring the solar flux wasspecially created in order to demonstrate the thermochromic transitionof the films in the near infra-red band. The principle has been thesubject of a publication in an international review (J. C. Valmalette etal., Solar Energy Materials, 1994).

The artificial solar source (11) is constituted of a 50W halogen lampwhose emission maximum is centred on 1 μm. The samples are compositefilms or coatings (13) of 58 mm diameter deposited on a glass substrate(16) placed against a source (11) and the detector (10) which measuresthe luminous flux for wavelengths between 0.3 and 2.8 μm. The multimeter(15) measures the voltage delivered by the detector (10). Each samplemay be heated or cooled by an air flow (12) and the temperature of thefilm is measured with the aid of a thermocouple linked to thethermometer (14).

The exploitation of the experimental results allows having access tothree optical scales directly linked to the method of manufacture of thefilm and to the quality of the transition.

The characterisation of each of the films comprises:

a direct measurement of the radiation of the source (without samples),

a standardisation from a semi-opaque film which comprises anon-thermo-chromic black pigment by a high and low temperaturemeasurement.

a measurement on a glass plate alone I°

a measurement of the cold film (T<T_(t)): I_(cold)

a measurement of the hot film (T>T_(t)): I_(hot).

Three scales have been defined by the calculation (in the solar spectralrange of the detector):

the opacity

the relative efficiency (1−(I_(hot)/I_(cold)) expressed in %

and the absolute efficiency (I_(hot)−I_(cold)) in standard units: W.m⁻².

The results obtained are the following (the values of flux transmittedin the cold are expressed as a function of an incident radiation of1,000 W.m⁻²).

Dry film of 10 μm thickness which contains a mass fraction of non-dopedVO₂ equal to M. F.=0.01.

opacity=34±2%

Flux transmitted in the cold (T<T_(t)=66° C.)=662 W.m⁻²

transmitted in the hot (T>T_(t)=66° C.)=606 W.m⁻²

Relative efficiency=8.5%

Gain in absolute efficiency=56 W.m²

Dry film of 10 μm thickness which contains a mass fraction of non-dopedVO₂ equal to M. F.=0.025.

opacity=40±2%

Flux transmitted in the cold (T<T_(t)=66° C.)=631 W.m⁻²

Flux transmitted in the hot (T>T_(t)=66° C.)=527 W.m⁻²

Relative efficiency=16.5%

Gain in absolute efficiency=104 W.m⁻²

Dry film of 10 μm thickness which contains a mass fraction of non-dopedVO₂ equal to M.F.=0.05.

opacity=63%

Flux transmitted in the cold (T<T_(t)=66° C.)=270 W.m⁻²

Flux transmitted in the hot (T>T_(t)=66° C.)=186 W.m⁻²

Relative efficiency=31.1%

Gain in absolute efficiency=84 W.m⁻²

Dry film of 100 μm thickness which contains a mass fraction of VO₂ dopedwith 1% tungsten equal to M.F.=0.005±0.001.

opacity=68±2%

Flux transmitted in the cold (T<T_(t)=66° C.)=708 W.m⁻²

Flux transmitted in the hot (T>T_(t)=66° C.)=635 W.m⁻²

Relative efficiency=31.1%

Gain in absolute efficiency=73 W.m⁻²

These results show that the volume fraction of pigment has a directincidence upon:

the thermal gain during their transition,

the opacity of the varnish sheet.

EXAMPLE 8 Particle Size Study

This study was carried out by counting on an electron microscope onseveral samples of films made with non-doped VO₂ obtained according toExamples 1 and 2. The films were prepared according to the method ofExample 7.

The results are given in Tables 1 and 2 below.

TABLE 1 Films containing non-doped VO₂ according to Example 1 .normallog distribution centred on D* = 0.3 μm with a standard deviation Ln σ =60 .film thickness = 10 μm .volume fraction (volume of the pigment/totalvolume of the film) = 0.02 Sizes (microns) % of the total Average size DPopulation D population (microns) A <0.16 0.158 0.1 B 0.16-0.55 0.6840.3 C >0.55 0.158 1 D = diameter of the particles; D = average diameterof the particles

TABLE 2 Films containing non-doped VO₂ according to Example 2 .normallog distribution centred on D* = 1 μm with a standard deviation Ln σ =0.8 .film thickness = 10 μm .volume fraction (volume of thepigment/total volume of the film) = 0.02 Sizes (microns) % of the totalAverage size D Population D population (microns) A <0.2 23 0.17 B0.2-0.45 13.6 0.30 C 0.45-1    34.2 0.67 D   1-2.23 34.2 1.5 E 2.23-5   13.6 3.34 F >5   23 5.85 D = diameter of the particles; D = averagediameter of the particles

What is claimed is:
 1. Microparticles of vanadium dioxide of formulaV_(1−x)M_(x)O₂ in which 0<x<0.05 and M is a doping metal, according towhich said microparticles have a particle size of between about 0.1 μmto about 0.5 μm.
 2. Microparticles according to claim 1 of formulaV_(1−x)M_(x)O₂ in which M is a metal selected from the group consistingof Nb, Ta, Mo and W.
 3. Microparticles according to claim 1 of formulaV_(1−x)W_(x)O₂ in which x is between 0 and 0.02.
 4. A method forobtaining particles of vanadium dioxide of formula V_(1−x)M_(x)O₂ inwhich M is a doping metal and 0≦x≦0.05, by pyrolysing doped or non-dopedammonium hexavanadate, according to which said pyrolysis is carried outat a temperature between about 400° C. and about 650° C., with atemperature increase rate of at least 100° C./minute, and in that thegases resulting from said pyrolysis are kept in confinement and indirect contact with the reaction medium for a period of time of at least½ hour.
 5. The method according to claim 4, said method being carriedout with ammonium hexavanadate doped with a metal selected from thegroup consisting of Nb, Ta, Mo and W.
 6. The method according to claim4, according to which the temperature increase rate is at least 200°C./minute.
 7. The method according to claim 4, according to which theduration of the confinement of the gases resulting from the pyrolysis isat least 5 minutes.
 8. The method according to claim 4, according towhich, before pyrolysis, the ammonium hexavanadate is submitted to adegassing at a temperature below 230° C. and in carrying out a firstpumping under vacuum for at least 1 minute.
 9. The method according toclaim 4, according to which, after the pyrolysis, the vanadium dioxideobtained is submitted to an optional annealing step under inert gas at atemperature of at least 600° C. for a period of time of at least 1 hour.10. The method according to claim 4, according to which, after thepyrolysis the vanadium dioxide is cooled under inert gas to atemperature of about 120° C.
 11. The method according to claim 4,according to which the vanadium dioxide obtained is optionally submittedafter the pyrolysis, said pyrolysis being optionally followed by anannealing step and a cooling step to a grinding.
 12. Surface coatingcompositions which contain microparticles according to claim
 1. 13. Themethod according to claim 4 wherein the period of time is one hour. 14.The method according to claim 6 wherein the temperature increase rate isat least 300° C. per minute.
 15. The method according to claim 7 whereinthe duration of the confinement of the gases is between one half hourand two hours.
 16. A method for preparing a surface coating compositioncomprising incorporating microparticles of vanadium dioxide of formulaV_(1−x)M_(x)O₂ in a mixture comprising a pigment or a varnish, in which0<x<0.05 and M is a doping metal, and in which said microparticles havea particle size of between about 0.1 μm to about 0.5 μm.
 17. Themicroparticles of claim 1, wherein the microparticles are substantiallytransparent.
 18. The surface coating composition of claim 12, whereinthe microparticles do not significantly change the color of the surfacecoating composition.
 19. The surface coating composition of claim 12,wherein the microparticles do not significantly change the transparencyof the surface coating composition.